Inertial and electromagnetic tracking are the two main methods of tracking mobile platforms (MP) such as airplanes, missiles, boats and cars. An electromagnetic tracking system (EMT) is typically based on estimating the direction of the maximum electromagnetic intensity at the receiver. Step track, conical scan and monopulse are examples of EMT methods.
An antenna typically has a beam width ranging from a fraction of a degree to several degrees. While this is sufficiently accurate for some tasks such as locating the target, it is not accurate enough for other tasks. In step tracking which is also referred to as hill climbing, the signal location is assumed known within the uncertainty of the antenna's main beam and the antenna is initially pointed at the estimated signal location. The antenna is then open loop commanded by equal and opposite angular displacements from this estimated signal location, e.g. in the azimuth direction, and the received signal level is measured at both angular displacements. Likewise, the antenna is also open loop commanded by equal and opposite angular displacements in the orthogonal plane, e.g., the elevation direction, and again the signal level at each displacement is again measured. If the signal level in each plane is identical at both angular displacements, the antenna is correctly boresighted with the signal. Differences in the signal level at the two angular offsets can be used to realign the antenna so that the boresight axis is coincident with the signal path direction.
Conical scanning is a method used to properly steer the antenna to track an MP. In this case, the antenna is continuously rotated at an offset angle relative to the tracking axis, or has a feed that is rotated about the antenna's tracking axis. As the beam rotates around the tracking axis beam returns from the MP are measured. Considering the case in which the MP is not aligned with the tracking axis, an amplitude modulation (AM) exists on top of the returned signal. This AM envelope corresponds to the position of the target relative to the tracking axis. Thus, the extracted AM envelope can be used to drive a servo-control system in order to align the target with the tracking axis. Typically, a conical scan system needs at least four MP beam returns to be able to determine the MP azimuth and elevation coordinate (two returns per coordinate).
Amplitude comparison monopulse tracking is similar to a conical scan in the sense that four squinted beams are typically required to measure the target's angular position. The difference is that the four beams are generated simultaneously rather than sequentially. For this purpose, a special antenna feed is utilized such that the four beams are produced simultaneously. Typically, four feeds, mainly horns, are used to produce the monopulse antenna pattern. When a mobile platform is located on the antenna tracking axis the four horns receive an equal amount of energy. However, when the target is off the tracking axis an imbalance of energy occurs within the different beams. This imbalance of energy is used to generate an error signal that drives the servo-control system. Typical monopulse processing consists of computing a sum and two difference (azimuth and elevation) antenna patterns. Then, by dividing a difference channel voltage by the sum channel voltage, the angle of the signal can be determined.
Electromagnetic tracking is involved with errors in estimating the MP state; some of the causes for electromagnetic (EM) errors are described as follows. Measurement of the return EM reference from a moving platform is not accurate and is sensitive to return EM intensity variations (e.g. as a result of airplane maneuvers). The multipath phenomenon is the propagation that results in radio signals reaching the receiving antenna by two or more paths. Causes of the multipath include atmospheric ducting, ionospheric reflection and refraction and reflection from terrestrial objects, such as mountains and buildings. The multipath effect causes changes in received EM intensity signal (which is often called scintillation or signal “breathing”) especially when the elevation angle of the antenna is close to the horizon as described schematically in FIG. 1. Ground station antenna 22 is directed towards moving target 24. As a result of the antenna low elevation angle, signal returns are obtained not only from direct path 26, i.e. the line of sight (LOS) to the target, but also from other directions 28 a result of the multipath phenomenon, reflecting off surface objects 30.
Navigation systems (NSs) on board moving platforms (MPs) are installed typically on board spacecrafts, missiles, aircrafts, surface ships, submarines or land vehicles. Typical NSs in use are inertial navigation systems (INS), global positioning systems (GPS) and star trackers. The INS typically consists of an inertial measurement unit (IMU) containing a cluster of sensors such as accelerometers and gyroscopes, which measure the platform linear acceleration and angular velocity respectively. Navigation computers calculate an estimate of the position, velocity, attitude and attitude rate of the mobile platform (starting from known initial conditions). This is achieved by integrating the sensor measurements, while taking into account the gravitational acceleration. INS suffers from integration drift, as small errors in measurement are integrated into progressively larger errors in velocity and especially position. This is a problem that is inherent in every open loop control system. The INS is inherently well suited for integrated navigation, guidance and control of host MPs. Its IMU measures the derivative of the variables to be controlled (e.g., position velocity and attitude). The INS is typically autonomous and does not rely on any external aids or on visibility conditions. It is therefore immune to jamming and deception. An inertial tracking system (ITS) which is usually based on INS, computes the relative change in position and orientation from the appearing acceleration and angular velocity in the MP with respect to an inertial reference coordinate system as illustrated schematically in two dimensions in FIG. 2 to which reference is now made. Solid arrows 40 and dashed lines 42 represent the MP computed and actual orientation vectors respectively, filled 44 and unfilled 46 circles represent the computed and actual MP position respectively.
With a known absolute start position p0 and start orientation vector q0 at time T0 the orientations vectors q1, q2 and positions p1, p2 at time T1 and T2 respectively are determined. The inertial tracker computes the relative changes in position Δp1 Δp2 and orientation Δq1,Δq2 and from the start configuration the actual position and orientation is determined.
The MP tracking errors such as position, orientation, velocity and acceleration in both methods cause degradations in tracking performance. Such degradations are noticeable, for example, when using a narrow beam antenna to track an MP, in such a case an accurate tracking system is needed to pin-point an MP. In another example, a narrow beam antenna is pointed towards an MP. When the antenna's axis is aligned exactly with the line of sight (LOS) between the antenna and the MP, a strong signal is detected. As the tracking error increases (i.e. the antenna axis is shifted with respect to the line of sight) the signal power decreases proportionally to the tracking error (within the limits of the main lobe). This power loss should be taken into account in power link budget calculations.